The Red Sox used Justin Garza as an opener against the Twins on Thursday because their rotation has been hit hard by injuries (Tanner Houck, Chris Sale) and ineffectiveness (Nick Pivetta, Corey Kluber). Abbie Parr/Associated Press

CHICAGO — The Red Sox are down to just four starters on their active roster after Tanner Houck (facial fracture) joined Chris Sale (left shoulder inflammation) on the injured list.

Boston used Justin Garza as an opener in Houck’s spot Thursday, then went to Brandon Walter (MLB debut) as the bulk reliever and lost 6-0 to the Twins.

The Red Sox are unsure how they will fill their empty rotation spot moving forward, which might come up again Wednesday despite the day off Monday.

Boston plans to start Garrett Whitlock on extra rest Tuesday. Brayan Bello could have started on regular rest Wednesday but the Red Sox have scheduled him for Thursday to give him an extra day. They have Wednesday’s starter listed as TBD, so they might need to use an opener again that day.

The Red Sox are in this difficult situation after it appeared they had considerable starting pitching depth for the first couple of months. They were at one point using a six-man rotation.

They even optioned Bello back to Triple-A Worcester on April 24 for a brief time because they had five others in the rotation (Houck, Whitlock, Sale, Nick Pivetta and Corey Kluber). They also had James Paxton on the verge of returning.

“It just goes to show there is no such thing as having enough pitching,” Chief Baseball Officer Chaim Bloom told MassLive on Saturday at Guaranteed Rate Field.

Injuries and ineffectiveness depleted the depth. Paxton’s return sent Pivetta (6.30 ERA, eight starts) to the bullpen. The Red Sox don’t plan to put Pivetta back into the rotation because he’s found success in his new role. He has a 2.16 ERA in 11 relief outings (16 2/3 innings).

“Some of why our starter depth isn’t what it was is because these guys are helping us in significant roles out of the pen,” Bloom said.

Chris Murphy, who started the year as a depth starter in Triple-A Worcester, also has found success since moving to the bullpen. Like Pivetta, the Red Sox will continue to use him as a reliever.

“The way we sit right now, that’s the best place for them and it’s the best place for us to have them,” Bloom said. “So that’s not a bad thing but obviously it’s us and 29 other clubs that get to this point in the season and I don’t know any team that is happy or necessarily satisfied with the pitching depth.”

Corey Kluber struggled both as a starter and a reliever for the Red Sox and is now on the injured list. Brandon Sloter/Associated Press

The Red Sox also demoted Kluber to the pen last month when Whitlock returned from the IL. Kluber posted a 6.26 ERA in nine starts. He continued to get hit hard as a reliever with a 9.45 ERA and six homers allowed in 13 1/3 innings. Boston placed him on the injured list earlier this week because of right shoulder inflammation. So he’s not an option to return to the starting rotation either.

There’s not much else at Worcester beyond Walter. Prospect Shane Drohan, who is not on the 40-man roster, needs more development time at Triple A. He has walked too many batters since being promoted from Double-A Portland. Bryan Mata entered the season as a depth starter but has been on Worcester’s injured list since early May with a shoulder injury.

It would be a great story if Rio Gomez – a 2017 36th round pick and son of late ESPN reporter Pedro Gomez – made it to the big leagues. But he has started just nine games in his minor league career, five this season. He also had never pitched in Triple A until earlier this month.

Jake Faria, who is not on the 40-man roster, is an option at Worcester with some big league starting experience. The Red Sox also recently signed veteran Kyle Barraclough, who has major league experience, to a minor league deal out of independent ball.

Do the Red Sox need to go outside the organization to find a starter, perhaps make a trade?

“You have to constantly be on the lookout to add it and constantly pushing yourself to see if you can get more out of the guys that are in-house because if you can help somebody take that leap to where they really become a viable part of that mix, it just gives you that many more options moving forward,” Bloom said.

Bloom said the trade “discussions around the league are starting to happen.”

“One of the things I’ve learned in this new calendar with where the draft is now (at the All-Star Break instead of June), you see these discussions start to happen now in June and then for the most part they go on a little bit of a hiatus around the draft,” Bloom said. “When the draft is over, now all of a sudden, it’s deadline season. But obviously again, it’s a need that 30 clubs have. So many clubs are in the hunt that it’s not something that is going to be easy to go out and get. There obviously will be guys available. But because it’s a need for everybody, they’re going to be hotly contested.”